This invention relates to a process for zinc electroplating of aluminum strip. More particularly, it is concerned with a process for preparing zinc- or zinc alloy-plated aluminum strip which has a plated coating having good adhesion to the aluminum substrate and which is suitable for use in the manufacture of automobile bodies by continuous direct electroplating at a high speed.
Aluminum sheet has begun to be employed in automobile bodies for the purposes of saving weight and thereby reducing fuel consumption. It is known that aluminum sheet which has been plated with zinc or a zinc alloy is suitable for use in such applications, since chemical conversion treatment such as phosphating or chromating can be easily performed on such plated aluminum sheet prior to finish paint coating. See Japanese Patent Application Laid-Open (Kokai) No. 61-157693 (1986).
However, there was no commercial process established in the prior art for direct electroplating of aluminum strip with zinc or a zinc alloy in a continuous manner at a high speed.
Aluminum and its alloys have high surface activity and form on the surface thereof a firm oxide film which is readily regenerated after removal. The presence of such an oxide film on the surface significantly inhibits the adhesion of a plated coating formed thereon. Therefore, when aluminum strip is electroplated, it has been considered necessary in the prior art to subject the aluminum strip to special pretreatment in order to remove the oxide film prior to electroplating.
For this purpose, displacement plating (also called immersion plating) with zinc or a zinc alloy is widely employed. This pretreatment method comprises forming a thin layer of zinc or a zinc alloy such as a Zn-Ni, Zn-Cu, or Zn-Fe alloy on the surface of aluminum strip (which is made of aluminum or an aluminum alloy) by means of displacement plating before the desired zinc electroplating is performed. The pretreatment method is performed by a process comprising the following steps, for example:
Degreasing with an organic solvent.fwdarw.Alkaline degreasing .fwdarw.Rinsing.fwdarw.Etching.fwdarw.Rinsing.fwdarw.Acid dipping.fwdarw.Rinsing .fwdarw.First Zn or Zn alloy displacement plating.fwdarw.Rinsing.fwdarw.Acid dipping.fwdarw.Rinsing.fwdarw.Second Zn or Zn alloy displacement plating.fwdarw.Rinsing.fwdarw.Strike Co or Ni plating. PA0 (1) 120 g/l of sodium hydroxide, 20 g/l of zinc oxide, 2 g/l of crystalline ferric chloride, 50 g/l of Rochelle salt, and 1 g/l of sodium nitrate, 21.degree.-24.degree. C., immersion period of 30 seconds. PA0 (2) 120 g/l of sodium hydroxide, 20 g/l of zinc oxide, 1-2 g/l of nickel cyanide, and 1 g/l of cuprous cyanide, 27.degree.-30.degree. C., immersion period of 20-60 seconds. PA0 (3) 500 g/l of sodium hydroxide, 100 g/l of zinc oxide, 1 g/l of crystalline ferric chloride, and 10 g/l of Rochelle salt, 16.degree.-27.degree. C., immersion period of 30-60 seconds.
The Zn or Zn alloy displacement plating is performed by immersing aluminum strip in a plating bath. Examples of compositions of useful plating baths and the immersion conditions are as follows:
Pretreatment of aluminum strip by such a displacement plating method involves the following problems.
(a) The displacement plating is performed twice, leading to an increased number of steps. Furthermore, the Zn or Zn alloy plated coating formed by the first displacement plating is dissolved out into an acid in the subsequent acid dipping step prior to the second displacement plating. Therefore, it is a waste of resources and increases the costs required for waste water treatment.
(b) The plating bath used in each displacement plating is an alkaline bath containing a toxic substance such as a cyanide or Rochelle salt and requires more complicated bath control than an acidic plating bath such as a sulfate bath.
(c) It takes a relatively long period of 20-60 seconds to complete the desired displacement in each displacement plating step. As a result, an overall treating period of from about 3 minutes to about 13 minutes is required to proceed from the solvent degreasing step to the second displacement plating step. Therefore, the pretreatment method significantly interferes with the production efficiency.
Consequently, when zinc electroplating is applied to aluminum strip in a continuous plating line in which the aluminum strip is pretreated by the above-described method prior to the desired electroplating, it is impossible to attain a high line speed and a high efficiency as realized in a similar continuous electroplating line for steel strip. If a continuous electroplating line for aluminum strip having a line speed as high as that employed in electroplating of steel strip is constructed, it will have a line length which is several times as long as the length of an electroplating line for steel strip.
Accordingly, there is a need for a direct zinc electroplating method for aluminum strip which eliminates the pretreatment procedure comprising displacement plating.
Japanese Patent Application Laid-Open (Kokai) No. 51-64429(1976) describes pretreating aluminum strip by anodizing so as to form an anodic oxide film containing a heavy metal such as copper as an impurity, which serves as a nucleus in the subsequent electrodeposition of zinc plating. This pretreatment, however, requires 10 to 45 minutes to complete anodizing and significantly interferes with the efficiency of plating.
Japanese Patent Publication No. 57-20399(1982) discloses a process for electroplating aluminum strip which comprises immersing aluminum strip in an alkaline solution or a hydrofluoric acid-containing acidic solution and then treating it in a mixed acid to roughen the surface of the strip before the strip is electroplated. According to that process, the oxide film formed on the surface of the aluminum strip is removed by immersing the strip in the alkaline or acidic solution and the surface is then roughened by dissolution with the mixed acid in order to assure good adhesion of a plated coating formed in the subsequent electroplating step to the aluminum strip substrate.
Also in that process, it takes a long pretreatment period of 55-165 seconds to remove the surface oxide film and roughen the surface. Therefore, the process is not suitable for continuous plating at a high speed since it requires a long plating line. In an example in that Japanese Patent Publication, aluminum strip is electroplated with zinc using a borofluoride bath. However, the current density employed in that example is very low, i.e., on the order of 6 A/dm.sup.2. Accordingly, although the process is a kind of direct plating, it does not provide a high-speed, continuous plating process.